As synthetic hair fiber, halogen-containing synthetic fibers such as modacrylic fiber and poly(vinyl chloride) fiber with excellent flame retardancy are widely used. However, because these halogen-containing synthetic fibers have low heat resistance, by dyeing at high temperatures and for long hours damage by heat such as frizz and shrinking of the ends tends to occur. Multi-color dyeing especially tends to damage fibers as it repeats dyeing. In order to reduce damage by heat, methods to evenly dye at a low temperature or in a short period have been considered. For example, in JP-A-57-16981, a method of using a carrier (accelerating agent) has been disclosed, and in JP-A-10121385, a method of using a solvent such as acetone has been disclosed. Although both enable dyeing in a short period, fastness (lightfastness, color fastness to rubbing) is insufficient and, when used in large amounts, there is the problem of abnormal -shrinking (frizzing) or embrittlement of the fiber due to swelling. Furthermore, in JP-A-01-174683, a method for multi-color dyeing of synthetic hair fiber is disclosed, but because the fiber is dipped multiple times into the dyeing solution at a temperature of 80 to 90° C., which is lower than usual, there is the problem of the dyeing process taking a long time.
The present invention solves the problems of the prior art. That is, the present invention provides a process for preparing synthetic hair fiber in which the fiber can evenly be dyed at a low temperature in a short period while maintaining excellent fastness without abnormal shrinking (frizzing) or embrittlement of the fiber due to swelling.